Single-carrier frequency division multiple access (SC-FDMA) is a frequency-division multiple access scheme. Like other multiple access schemes (TDMA, FDMA, CDMA, OFDMA), multiple users are assigned to a shared communication resource. As in Orthogonal Frequency-Division Multiple Access (OFDMA) systems using Orthogonal Frequency-Division Multiplexing (OFDM) techniques, guard intervals with cyclic repetition are introduced between blocks of symbols in view to efficiently eliminate time spreading (caused by multi-path propagation) among the blocks.
Unlike OFDMA systems, in SC-FDMA, data symbols are transmitted sequentially. The modulation symbols are spread so that each subcarrier is carrying a part of each modulation symbol. The SC-FDMA symbol period is the same length as the OFDMA symbol but due to sequential transmission, the data symbols in SC-FDMA are shorter. The symbols occupy one OFDM symbol period divided by the number of sub-carriers. A consequence of the higher data rate symbols is that greater SC-FDMA signal generation bandwidth is required, so each data symbol occupies bandwidth corresponding to the number of sub-carriers times the OFDM bandwidth.
Multiple access among users is made possible by assigning to different users different sets of non-overlapping Fourier-coefficients (sub-carriers). This is achieved in known SC-FDMA schemes at the transmitter by inserting, prior to inverse Fast Fourier Transform (IFFT) processing, silent Fourier-coefficients (at positions assigned to other users), and removing them on the receiver side after the FFT.
A feature of SC-FDMA is that it leads to a single-carrier transmit signal, in contrast to OFDMA, which is a multi-carrier transmission scheme. Owing to its inherent single carrier structure, a prominent advantage of SC-FDMA over OFDM and OFDMA is that its transmit signal has a lower peak-to-average power ratio (PAPR). This is due in part to the fact that while in OFDM the transmit symbols directly modulate the multiple sub-carriers, in SC-FDMA the transmit symbols are first preprocessed by a DFT block.
During signal transmission using both OFDMA or SC-FDMA, out-of band emission represents a challenge. The out-of-band emission refers to signal power leakage out of the desired signal bandwidth, and is due to factors such as imperfections in transmit and receive filters. Moreover, the non-linearity of a power amplifier (PA), which causes distortion of the waveform during the conversion of a low power radio frequency (RF) signal into a high power RF signal, may also result in unwanted spectral emission into an adjacent frequency band. Because SC-FDMA may have a lower PAPR compared to OFDMA, the power amplifier backoff required to preserve linearity may be less. Thus, SC-FDMA has attracted interest as an attractive alternative to OFDMA, especially in the uplink communications where lower PAPR greatly benefits the mobile terminal, or user equipment (UE), in terms of transmit power efficiency and terminal costs. It is currently the designated technology for uplink multiple access schemes in 3GPP Long Term Evolution (LTE), or Evolved Universal Terrestrial Radio Access (E-UTRA), and for uplink wireless transmission in future mobile communication systems where transmitter power efficiency is of paramount importance.
For UE transmitters used to transmit SC-FDMA signals, it may therefore be desirable to reduce the PAPR while still meeting in-band signal requirements imposed by the LTE standard. Known techniques for reducing the PAPR of the baseband (BB) signal include a digital “clipping” (which may involve limiting magnitude of the signal samples while preserving their phases) process, followed by processing using a digital low pass filter (LPF) to suppress the spectral leaks resulting from the clipping. However, the LPF filtering may have the adverse effect of increasing the peak magnitude, and consequently, the PAPR, of the signal, thereby reversing at least part of the PAPR reduction done by the clipping. It is with respect to these and other considerations that the present improvements have been needed.